FIG. 1 illustrates a typical prior art earth-boring bit 11. Bit 11 has a bit body 13 that is threaded at its upper end for connection into a drill string. Bit body 13 has a number of pressure compensating lubricant reservoirs 15. Bit body 13 is also provided with at least one nozzle 17, which discharges drilling fluid from down the drill string to cool bit 11 and wash cuttings produced during drilling out of the borehole.
A plurality of cones 19, 21 are mounted for rotation on cantilevered bearing pins. In this prior art example, there are three cones, but only two are shown. Cones 19, 21 are shown with a plurality of teeth 23, each having a crest 25 that extends parallel with an axis of rotation of each cone 19, 21. During drilling operation, cones 19, 21 roll over the bottom of the borehole being drilled while teeth 23 penetrate and disintegrate the earth's formation.
Prior art bits similar to that illustrated in FIG. 1 have a shortcoming that becomes particularly apparent during drilling of formations, such as shales, that behave plastically. During drilling of these formations, conventionally arranged teeth 23 tend to fall into indentations made by the same or another tooth 23 on a previous revolution of bit 11. This condition is known as tracking and can seriously impair the penetration rate, life and performance of bit 11.
Another shortcoming of the prior art bit illustrated in FIG. 1 is that formation material may become packed between teeth 23, preventing teeth 23 from penetrating the formation deeply and thereby reducing the rate of penetration of bit 11. This condition is known as balling. Balling, like tracking, prevents the teeth 23 from penetrating to fall depth, thus resulting in inefficient and costly drilling. Balling also prevents the force on crests 25 of teeth 23 from reaching the level sufficient to fracture rock.
The characteristics of both tracking and balling are well recognized, but generally are treated as independent problems. In many cases, features that reduce tracking promote balling, and vice versa. For example, balling is more likely to occur between closely spaced teeth. Large and widely spaced teeth are more prone to tracking.